A traditional method for estimating the remaining capacity of a rechargeable battery is illustrated as following. FIG. 1 shows a system with a battery as a power source. The system includes an analog-to-digital converting (ADC) unit (2) to detect the voltage, current, and temperature of a battery; a battery unit (1) that provides power to the system; a micro processor unit (3) for processing the detected voltage, current, and temperature of battery (1) to calculate the remaining capacity of the battery; an operation device unit (4) for providing a charge source and a discharge load. The micro processing unit evaluates the remaining capacity of the battery according to the detected voltage (V), current (I) and temperature (T) and outputs the result to the system.
The capacity of a battery is represented by the quantity of electric charge; for example, a battery with capacity of 400 mAh indicates that the battery can discharge a current flow of 4000 milliampere continually for one hour (current*time=quantity of electric charge). The micro processor unit continuously reads the current during charging period (positive current) and discharging period (negative current) to calculate a cumulative input or output quantity of electric charge of the battery a “Coulomb calculation.” At a predetermined charging/discharging terminal conditions (e.g. 4.2V as charging terminal condition and 3.0V as discharging terminal condition), the output quantity of electric charge of battery is calculated at a controlled temperature (usually 25 Centigrade) and loading during a full charging/discharging cycle to define the rated (manufacture design) capacity of the battery.
In a practical application, the calculation of the remaining capacity of a battery becomes more complex and difficult due to environment conditions (e.g. load or temperature may change during operation) and chemical characteristics of the battery will decay (e.g. aging problem). Consequently, an inaccurate calculation makes the reminder of battery status less reliable. For example, charging/discharging a battery for a long time causes the aging problem and the residual capacity of the battery is hard to calculate accurately. The chemical characteristics of the battery, which vary with time (i.e. aging) and are affected non-lineraly by the discharging current, voltage, and temperature further increase the difficulty in calculating the remaining capacity. In addition to the Coulomb calculation, compensative methods for calculating the remaining capacity of battery are generally classified into the following two groups:
A. Discharge Condition Lookup Table
A lookup table is prepared based on an experimental simulation of a system to obtain the absolute residual capacity at different discharge currents, voltages, and temperatures. When a real system uses a battery as a power source, then the residual capacity can be obtained from the lookup table based on the measured voltage, current and temperature. The drawbacks of the method are bothersome experiments and the inaccuracy due to aging of the battery which experimental data still does not include.
B. Internal Resistance Calibration
The relationship among the battery internal resistance, the residual capacity, and temperature is determined experimentally beforehand. The system calibrates the residual capacity according to the detected battery internal resistance. Since the internal resistance may reflect the aging problem of the battery, the result is relatively accurate. However, an accurate measurement of the internal resistance is difficult and usually interferes with the charging/discharging process. Further, bothersome experiments need to be done to obtain the data table that corresponds to the relation between the internal resistance and the residual capacity of this type of battery.
On the other hand, for a system requiring less accuracy of remaining capacity, a rough indication is usually implemented, where such as a four level indication, each level represents 25% capacity. In this way, the capacity roughly corresponds to the voltage of battery. For example, a typical lithium battery with 4.2 volt corresponds to 100% capacity while 3.0 volt corresponds to 0% capacity. However, this rough indication still lacks of considerations of the aging problem, and the variation of current or temperature. Therefore, the calculation of the residual capacity using this method includes significant inaccuracy and is inconvenient for practical applications. Accordingly, improved methods of measuring a remaining capacity of a rechargeable battery are desired.